stm32f3: Some additions to rcc.

- Additional frequency configuration (48Mhz, for usb use!)
- FLASH latency decreased (too unnecessarily low before)
- Rcc functions to change usb freq prescaler.
stm32f3: Usb support added. usb unit is the same as f103.

- memorymap value for usb base changed to the one expected by the
  f103 usb code.
- f3 Makefile updated to build the f102 usb code.
stm32f3: f1 usb header files moved to stm32 root directory.

- Now the f1 usb header files serve both the f1 and f3.
- usb_f103 modified to find the new headers file location.